1. Field of the Invention
The present invention is directed generally to a lighting system having an electric lamp controlled by an infrared remote controller and, in particular, to an improved filter in an electric lamp remote control receiver which filters infrared noise generated by an electric lamp.
2. Description of the Prior Art
A typical remote control system includes a transmitter for transmitting information through modulation of an infrared ("IR") signal having a certain carrier frequency, and a receiver for receiving the signal and either creating an output signal or performing some function. The information in the modulated transmission signals typically includes leader code, logic 1's and logic 0's which are each defined, in one type of modulation, by the duration of time 21, 22 between bursts 20 of carrier pulses as shown in FIG. 3(a), or in other words, by the pulse period. An infrared receiver receives the modulated transmission signal through a photo diode, the signal is then sent through a pre-amplifier level control block, which acts as a gain control. A limiter clamps the amplitude of the modulated signal and the modulated signal is filtered by a bandpass filter which filters out noise having a frequency outside the bandwidth of the bandpass filter. The signal is also demodulated, amplified and then shaped into a square wave by a waveform shaping block. The pulse width 21a, 22a of the square wave as shown in FIG. 3(b) corresponds to the times 21, 22 between bursts of the transmitted signal and determine the type of data being transmitted. A decoder decodes the transmitted information from the square wave.
Infrared signals are affected by interference caused by various ambient sources such as the sun, incandescent and fluorescent lamps, infrared stereo headphone transmitters, etc. In the past, the interference associated with IR signals was attenuated by selecting a carrier frequency of the transmitted IR signal such that it was outside the range of ambient noise and by using various filters to filter out any signals having frequencies falling outside a band surrounding the chosen transmitting frequency. Such a design is known, for example, from the Signetics AN1731 Low-Power Remote Control IR Transmitter and Receiver Preamplifiers wherein the carrier frequency of the IR signal is 38 kHz and the receiver is a narrow band receiver which filters out TV line frequencies having harmonics falling outside a 3 kHz band surrounding the 38 kHz transmitted frequency. Because these types of band pass filters attenuate noise which falls outside the narrow frequency band of the filter a proper carrier frequency must be chosen which is outside the frequency range of ambient noise. If, however, there exists ambient noise at a frequency within the band of the bandpass filter, these types of filters will allow the ambient noise to interfere with the received transmission signal. The ambient noise which passes through the bandpass filter will be acted on by the various stages of the receiver and, depending on its intensity, duration, and frequency, will appear in the square wave output of the wave-shaper as extraneous leading and trailing edges within the duration of time required for a logic 0 or a logic 1 to be transmitted, as shown in FIG. 3c. These extraneous edges cause false decoding of the transmitted information from the square wave.
It has become increasingly more difficult to select a transmitting frequency outside the frequency range of all ambient noise. Infrared, i.e. noise generated by the sun and incandescent lamps have frequencies from 0 to 120 Hz, and noise generated by stereo headphone transmitters ranges from 95 kHz to 250 kHz with a frequency deviation of 50 kHz. Light transmitted from fluorescent lamps operated by high frequency dimming ballasts, which have only recently been made commercially available, has frequencies around 20-120 kHz. It has been found that the infrared component of the light from such high frequency operated fluorescent lamps will interfere with the transmitted 38 kHz signal frequency of the Signetics AN1731 remote controller, as the bandpass filter allows noise to pass having a frequency within the band of the bandpass filter. Thus, it is close to impossible to find a transmission frequency which is not the same frequency as some source of infrared noise, especially when operating in the vicinity of a fluorescent light controlled by a high frequency dimming ballast.
In U.S. Pat. No. 4,555,702, a remote control signal reproducing circuit is disclosed. This circuit will filter out the noise present in a modulated signal by counting the number of carrier pulses within a certain time interval and comparing the number counted to several predetermined values to determine whether a control signal is being received or whether noise is being received. Although U.S. Pat. No. 4,555,702 refers to these pulses as modulated signal pulses, they will be referred to as carrier pulses herein. Specifically, it considers the number of carrier pulses which would normally be present in a pulse width, i.e. if the carrier frequency is 40 kHz and the pulse width is 1 ms, then each burst in the transmission signal without noise would contain 40 carrier pulses within the 1 ms time period. If, however, noise is present, there may be only 30 carrier pulses since noise may cancel some of the pulses. It compares the number of pulses counted to several predetermined values, such as a minimum and maximum number of pulses for a valid pulse width, and decides whether a transmission signal is being received or whether noise is being received. It only assumes, however, that noise will subtract from the number of carrier pulses present rather than also adding to the number of pulses present, such as if the noise has a higher frequency than the carrier frequency.
The filter of U.S. Pat. No. 4,555,702 operates on the signal before the signal is demodulated. That is, the carrier pulses are present when the system removes noise. Most IR receivers, especially in the consumer electronics area, output a signal which is demodulated. Therefore, the filter disclosed in U.S. Pat. No. 4,555,702 cannot be used with these "off-the-shelf" receivers.